Concentrated solar power (CSP) plants typically provide dispatchable power by integrating thermal energy storage (TES) system, e.g., molten salt tanks with an electrical generator power block. However, thermochemical energy storage (TCES) in which energy is stored in chemical bonds, has advantages over TES. In particular, TCES systems may provide higher energy density, and experience lower energy losses during the storing operation. Ammonia-based TCES systems have the potential to be more efficient for long-term storage than compressed air energy storage, pumped hydroelectric energy storage, vanadium flow battery storage, or thermal energy storage.
In ammonia-based thermochemical energy storage systems for CSP, ammonia (NH3) is dissociated endothermically as it absorbs solar energy during daylight hours, producing supercritical hydrogen (H2) and nitrogen (N2). The stored energy can be released on demand when the H2 and N2 react exothermically to synthesize ammonia. The released thermal energy can then be used to generate electricity. A 1 kWsol closed loop ammonia-based TCES system and a 15 kWsol ammonia-based TCES system for dish power plants have been built and tested. The system demonstrated ammonia dissociation on a dish concentrator and subsequent energy recovery at temperatures high enough for electricity generation, but did not demonstrate heating of a working fluid. In particular, ammonia synthesis has not been shown to reach temperatures consistent with modern power blocks. For example, modern steam-driven Rankine cycle power blocks may be configured for supercritical steam heated to about 600° C., 650° C., or more.
A TCES system with a catalytic ammonia synthesis reactor is described below. In one embodiment the synthesis reactor is configured to operate at about 30 MPa. The effect of various parameters on the reactor performance has been investigated, to optimize the system for low-cost electricity generation.
For ease of expression herein, the water, which may be supercritical or liquid, is generally referred to as “steam.” The N2, H2, NH3 mixture is generally referred to as “the gas,” even though the species are all supercritical within the reactor and NH3 is liquid in the lower temperature portion of the system.